The present invention relates to a system of encoding digital signals comprising a source encoder followed by a transmission channel encoder. It also relates to a system of decoding digital signals which were previously submitted to a variable-length coding, an encoding with selective protection and an encoding without selective protection, the said system comprising a transmission channel decoder followed by a source decoder.
Digitizing television signals requires the possibility of transmitting a very large quantity of binary information components with a rate of the order of 220 Mbit/s. Such a rate cannot be ensured at reasonable cost by contemporary transmission channels, and different information encoding techniques have been proposed with the object of reducing the quantity of information components and consequently the rate. Such an objective is actually achieved by reducing the redundancy of information components, but, then, each information component transmitted becomes essential. Any possible transmission errors which might rather easily be corrected when the information components to be transmitted are redundant, have increasingly more serious consequences when this redundancy is reduced. Actually, the extent of faults due to transmission errors unfortunately increases more rapidly than the rate reduction factor.
When a transmission channel beset with noise is present, efforts have been made to find protection from these transmission errors or to reduce their effects. One of the techniques thus proposed consists, for the encoding of information components, in associating an error correction coding (alternatively denoted channel coding) to a rate reducing coding (alternatively denoted source encoding) which renders it possible to protect in a selective manner the information components which are most sensitive to transmission errors. A method and a coding system ensuring such a protection are described, for example, in the U.S. Pat. No. 4,555,729.
The recent use, in source encoders, of variable-length codes which still further improve the performances of these encoders, leads to a new reduction of the redundancy of the information components. Consequently, said information components are even more vulnerable to transmission errors. On the other hand, a variable-length encoding results in allocating to information component blocks of similar dimensions, a variable number of bits as a function of the information contained in each block. In this case, the presence of transmission errors may cause the loss of the proper segmentation of encoding words corresponding to a block, or the loss of synchronization between blocks, which would entail the appearance of false designs as well as spatial shifts in the picture.
These faults are difficult to correct with contemporary error correction techniques, to that extent that, with a variable-length encoding, the positions of important information components in the binary sequence are not known. An error in the most significant bits of the direct current component, for example, is much more perceptable than an error in the last bits of a block of information components. But, by reason of the variable length of the coding sequences (or words), the contemporary techniques have proved to be incapable of correcting this type of error in an adapted manner.